What is the best way to distort the shape of this leaf image plane? (i'm a complete beginner)
have not used Blender much in a long time either but hope this is what you're looking for.
I screen captured your leaf image used it as a reference to model a grid around it deleting the excess, made a material to map the image to the leaf, did a quick UV map , set displacement to bump, went into shading and changed settings to distort the surface.

rendered single leaf and it looked good , in object mode duplicated it nine times , took each leaf into edit mode &gt; sculpt mode and used the elastic deform tool to distort the shape from flat plane.

I rendered both sides this is the bottom so it seemed to work good.